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Towards the design of new nitrogen fixing symbionts

Elucidate how the capacity to establish a nitrogen fixing symbiosis with legumes has been acquired by new bacterial taxons via a lab-evolution experiment should allow developping the required know-how for the design of new symbionts.

Deciphering adaptive mechanisms leading to the rhizobium-legume symbiosis

Contrary to cereals and most cultivated plants, legumes do not require nitrogen fertilizers for growth thanks to their ability to enter a symbiosis with nitrogen-fixing microorganisms, called rhizobia. Symbiosis includes four major steps: nodule formation, bacterial infection, bacterial maintenance and nitrogen fixation. Recent major scientific advances have renewed interest and hope in bioengineering N2-fixing cereals. Bioengineering nodulating cereals will necessitate better understanding of key traits of the symbiotic interaction, i.e. nodulation competitiveness, intracellular infection, bacterial maintenance and nitrogen fixation as well as the parallel design of symbiotic bacterial partners adapted to these new plants. The SHAPE projecs aims at elucidating bacterial adaptive mechanisms leading to symbiosis as well as paving the way to experimental evolution-based strategies to design new rhizobia, e.g. adapted to new host plants.

We will take advantage of the previous experimental evolution of a pathogenic Ralstonia solanacearum chimera carrying the symbiotic plasmid of the rhizobium Cupriavidus taiwanensis into legume symbionts (in its primary sense «which lives with«) and will exploit the generated biological material evolved under legume selection pressure. The evolution of key phenotypic traits – including nodulation competitiveness and intracellular infection quality and persistence- along the evolution experiment will be analyzed using high resolution phenotyping at the cellular, transcriptomic and metabolic level. Experimental strategies to emerge N2-fixing Ralstonia, a trait not acquired along the evolution experiment. will be explored using the reference C. taiwanensis rhizobium Genomic changes and mechanisms underlying the adaptive process will be analyzed using whole genome resequencing (Illumina technology) and genetic reconstruction.